As a high-frequency signal level detection apparatus for detecting a signal level of a high-frequency signal received by an antenna or a cable, various kinds of apparatuses have been conventionally proposed, examples of which are disclosed in the following prior art documents.
(1) Japanese patent application laid-open publication No. 2002-217763 (hereinafter, referred to as a prior art document 1).
(2) Japanese patent application laid-open publication No. 9-199962 (hereinafter, referred to as a prior art document 2).
(3) Japanese patent application laid-open publication No. 60-062246 (hereinafter, referred to as a prior art document 3).
(4) Japanese utility model laid-open publication No. 62-093843 (hereinafter, referred to as a prior art document 4).
In each of these prior art documents 1 to 4, the signal level is basically detected based on an AGC (Automatic Gain Control) voltage that is a control voltage outputted from an AGC circuit that controls the signal level of a received high-frequency signal to be substantially constant.
The apparatus described in, for example, the prior art document 1 (hereinafter, referred to as a prior art apparatus) is characterized by generating a mapping function by correcting an AGC value by a predetermined value for a signal at a predetermined frequency or higher at which a change in the AGC value is greater according to characteristics of a high-frequency circuit block, storing the generated mapping function in a memory, correcting an AGC voltage by a predetermined value when a reception frequency at which an input level is displayed exceeds the predetermined frequency, and calculating a display level value of an inputted signal by the mapping function so as to reduce a display error in the display of the input level, and this leads to that an error may be caused due to a difference in reception frequency.